The invention relates to a circuit arrangement for converting digital signals, particularly PCM signals, into analog signals, particularly PAM signals, and for translating analog signals into digital signals. The arrangement exhibits an analog store accommodating analog signals which appear in pulse frames in cyclical repetition and which are to be converted into digital signals. The analog store has an assigned signal output device putting out such digital signals, a signal acceptance device accepting digital signals to be converted into analog signals, and also an analog-digital transducer operating in accordance with the iterative principle. The transducer has a flow control; an intermediate register which can be controlled by the control signals produced by said transducer; a digital analog converter connected at the outlet side of said intermediate register; and a comparator comparing the analog signals given off by the digital analog converter with the analog signals stored in the analog store and which are to be converted into digital signals. Output from the intermediate register is controlled by the flow control. The flow control is laid out such that it releases the transfer of a digital signal received by the signal accommodation device and to be converted into an analog signal to the digital analog converter, and effectively switches the digital analog converter for converting the analog signal stored in the analog store into a digital signal with specific other actuation signals. The inputs of an output register of the signal output device are connected to the register levels of the intermediate register, and the outputs of the input register of the signal acceptance device are connected to the placement inputs of the register levels of the intermediate register. Such a circuit arrangement (German Offenlegungsschrift 2,534,109) incorporated by reference herein merely requires one single analog digital transducer which is twice utilized within each pulse frame of the determining control impulses. This circuit arrangement is relatively inexpensive with respect to circuit layout since only one input register and one output register are required for the conversion of an analog signal into a digital signal and for the output of such a digital signal. The same is true for the accommodation of a digital signal to be converted into an analog signal and for the conversion of such a digital signal. The analog digital transducer can also be of conventional design.
The possibility of holding conference calls represents an often requested output characteristic, particularly in secondary or auxiliary exchange systems.
In telecommunication systems with analog connecting through, the conference sets required arranged in central position have a relatively simple circuit layout. If, however, as presumed in the present case, one deals with a PCM time multiplex telecommunication setup with digitally operating subscriber stations, a greater circuit layout expense must be made for the conference set. As the PCM words are coded according to a non-linear characteristic, the PCM words indeed cannot be readily added up to a summation word. Thus, for example, it is known for this purpose (German Offenlegungsschrift 1,918,100 corresponding to U.S. Pat. No. 3,612,772 and incorporated by reference herein) to convert the digital signals coming from the individual subscriber stations into analog signals, then to form an analog summation signal from the individual analog values and to finally transmit such a summation signal--after conversion into a digital signal--to the conference subscribers. Therefore within one pulse frame many summation signals are formed as conference subscribers take part in the conference call, whereby in each of the summation signals the call share of respectively one other of the conference subscribers is lacking.
The formation of these sums proceeds such that first the call shares of two subscribers are summed up respectively after one digital-analog conversion. The intermediate sum resulting therefrom is converted into a digital value and this intermediate sum again is converted back into an analog value and summed up with the call share of the next subscriber which was converted into an analog value, and so on, until finally a digital summation word is formed from the call shares of n-1 conference subscribers. This summation word is transmitted to the n-th subscriber.
The formation of the remaining summation words proceeds in a corresponding manner.
The known circuit arrangement cited requires three digital-analog converters, of which two are used for the digital-analog conversion before the intermediate summation or summation formation, and the third component of the coding device is used for the formation of the digital intermediate summation or summation signal. These digital-analog converters must operate very rapidly due to the multiple digital-analog conversion occurring within one pulse frame and are therefore correspondingly expensive.